This invention relates to a rotational torque adjustment apparatus for adjusting the rotational torque of a rotational member which rotates integrally with a rotational shaft, an ink ribbon transport apparatus wherein a rotational torque adjustment apparatus is used to supply an ink ribbon and a printer in which an ink ribbon transport apparatus is used.
Conventionally, in a printer in which an ink ribbon is used such as a sublimatic printer or a thermal transfer printer, if the ink ribbon becomes slackened upon printing, then a wrinkle may appear with the ink ribbon and give rise to a failure in printing. Therefore, generally back tension is applied to the ink ribbon to be fed out from the supply roll to prevent slackening of the ink ribbon.
Further, also on the takeup spool side, the takeup force is adjusted taking the back tension into consideration. Thus, the tension of the ribbon is controlled to a fixed level between the supply spool and the takeup spool.
Various measures are applied to the application of the ribbon tension, and measures which include a brake mechanism which includes a torque clutch, a torque limiter or a like element for each of the spools and different measures which use such a resilient member as, for example, a leaf spring or a like element are adopted principally.
However, as the winding diameter of the ink ribbon wound on the supply spool and the winding diameter of the ink ribbon taken up on the takeup spool gradually vary as printing proceeds, the torque applied to each of the spools varies. Therefore, various techniques have been proposed in order to apply a fixed level of ribbon tension to the ink ribbon.
For example, Japanese Patent Laid-Open No. 2002-234241 disclosed an apparatus wherein the rotational torque of a shaft for driving the takeup spool is determined by the clamping pressure of a clamping member and a spring compressing gear held in meshing engagement with the shaft is rotated to change the spring biasing force to the clamping member so as to adjust the rotational torque in response to the winding diameter of the ink ribbon.
Japanese Patent Laid-Open No. Hei 6-316139discloses another apparatus wherein the number of pulses generated in response to rotation of each of the takeup spool and the supply spool is counted and a motor voltage with which motor torque and a rotational speed suitable for the winding diameter of the ink ribbon is implemented is arithmetically operated to control the motor.
However, with the apparatus of Japanese Patent Laid-Open No. 2002-234241 described above, every time adjustment is to be performed, the compressing gear must be rotated to perform setting. Therefore, the apparatus has a disadvantage that much time is required for the setting.
Meanwhile, the apparatus of Japanese Patent Laid-Open No. Hei 6-316139 is disadvantageous in that the control is complicated and a mechanical system suffers from a dispersion caused by differences among different apparatus, resulting in the possibility that the stability in feeding of the ink ribbon may be degraded.